A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted electrode contact can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple stimulation contacts 112 on its surface that provide selective stimulation of the cochlea 104.
Most existing cochlear implant stimulation coding strategies represent a sound signal by splitting it into distinct frequency bands and extracting the envelope (i.e., energy) of each of these bands. These envelope representations of the acoustic signal are used to define the pulse amplitude of stimulation pulses to each electrode. The number of band pass signals typically equals the number of stimulation electrodes, and relatively broad frequency bands are needed to cover the acoustic frequency range. Each electrode contact delivers electric stimulation signals to its adjacent neural tissue for a defined frequency band reflecting the tonotopic organization of the cochlea.
Channel interactions between electrode contacts is caused by the electrically high-conductivity liquid inside the cochlea, and this causes spatial masking. FIG. 2 shows various spatial current spreads based on an exponential spread decay of different intensity on adjacent electrode contacts. An exponential current decay of 0.75 per electrode contact in both directions (apex and base) is assumed, and for convenience, only the spreads caused by the anodic stimulus phases are depicted in FIG. 2. The areas below a given current spread indicate approximately the amount of neuronal recruitments. Electrode contact masking occurs in the areas where current spreads of different electrode contacts overlap. The neurons in these regions are not exclusively related to a specific electrode contact and so are not independent.
When equal neuronal survival across the electrode contacts is given, on average, equal stimuli can be expected to produce equal loudness in a cochlear implant. The upper panel in FIG. 2 illustrates the case where two sequential stimuli on adjacent electrode contacts have the same pulse amplitude. In this case, the masking between these electrode contacts occurs to the same extent and is balanced so that the amount of overlap—the masked area—is equal in both current spreads. On the other hand, when pulse amplitudes of sequential stimuli are different (for equal loudness perception) as shown in the lower panel of FIG. 2, then the electrode contact masking is no longer balanced. The bottom two curves of the lower panel show the case where the current spread of electrode contact 7 is completely covered by the current spread of electrode contact 6. The top two curves of the lower panel show the case where the current spread of electrode contact 6 is completely covered by the current spread of electrode contact 7.
In general, the amount of masking between electrode contacts is a function of their spatial distance and current amplitude. Channels with higher current amplitude will mask neighbouring electrode contacts to some specific extent. The greater the difference in amplitudes between adjacent electrode contacts, the greater will be the resulting masking. If a given electrode contact is completely masked by other electrode contacts, then the related frequency band of that electrode contact cannot be perceived by the implanted listener and is thus missing.
In a conventional cochlear implant fitting procedure, the effectiveness of each electrode contact has to be judged by a skilled audiologist. Each electrode contact has to be assessed in terms of minimum Threshold level (THR) and Maximum Comfort Level (MCL) values, and then further judged in terms of usefulness by the audiologist in cooperation with the implant user (if feasible). The result of this process greatly depends on the skills of the audiologist and the implant user. Electrode contacts with extremely low or high MCL characteristics often are disabled to ensure a reasonable representation of all frequency bands of the sound processor.